Due to the scarcity of available spectrum for future mobile, wireless communication systems, spectrum located in very high frequency ranges (compared to the frequencies that have so far been used for wireless communication), such as 10 GHz and above, are planned to be utilized for future mobile communication systems, e.g. 5th Generation (5G), including the 5G system being standardized by 3rd Generation Partnership Project (3GPP), for which the Radio Access Network part is referred to as New Radio (NR) and the core network part is referred to as Next Generation Core (NGC).
For such high frequency spectrum, the atmospheric, penetration and diffraction attenuation properties can be much worse than for lower frequency spectrum. In addition, the receiver antenna aperture, as a metric describing the effective receiver antenna area that collects the electromagnetic energy from an incoming electromagnetic wave, is inversely proportional to the frequency. Consequently, if omnidirectional receive and transmit antennas are used, the link budget will be worse for the same link distance even in a free space scenario. This motivates the usage of beamforming to compensate for the loss of link budget, resulting in poor Signal to Noise Ratio (SNR)/Signal to Interference and Noise Radio (SINR), in high frequency spectrum (e.g. >g GHz). Beamforming may be used at the transmitter, at the receiver, or both. In a large part of the spectrum planned for 5G deployments the preferred configuration is to use a large antenna array at the Access Node (AN) (e.g. base station in New Radio (NR) (gNB) (a gNB corresponding to eNB in Long Term Evolution (LTE)), Transmit/Receive Point or Transmission/Reception Point (TRP), Evolved NodeB (eNB)) and a small number of antennas at the User Equipment (UE). Access Node (AN) is a generic term for a node in a cellular network or mobile communications network, serving a radio interface towards wireless terminals using the network. Other terms that may be seen as covered by the generic Access Node term include e.g. eNB, gNB, and TRP. The large antenna array at the AN enables high-order transmit beamforming in the downlink.
For the above reasons, future systems are expected to make heavy use of high-gain, narrow beamforming, which will enable high data rate transmission coverage also to very distant users which would not realistically be covered with normal sector-wide or omnidirectional beams, which have lower antenna gain.
High or medium gain beamforming has traditionally mostly been used to increase the achievable data rates for individual UEs. However, for 5G/NR, beamforming is expected to be used also for common control signaling, i.e. control signal transmissions that are not targeting a specific UE but is intended to be received by all or a group of UEs in the coverage area. Such common control signaling include e.g. synchronization signals, broadcast system information and common reference signals.
These signals, by nature, must reach the entire coverage area that a gNB (or possibly a group of TRPs) is intended to serve, e.g. a cell. To achieve this goal using beamformed transmissions a concept known as beam sweeping may be utilized, whereby the concerned signal is transmitted using sequential transmissions of narrow beams in different directions until the entire coverage area—e.g. cell—is covered. Another way of combating the low SNR/SINR is to use repeated wide (or omnidirectional) beam transmissions, which the UE soft-combines until enough energy has been collected to achieve good enough SNR/SINR to allow decoding of the information.
Synchronization Signals and System Information in NR
Of special interest in the context of the present disclosure are synchronization signals and system information and how these are transmitted in NR. The synchronization signal consists of two or possibly three components:                Primary Synchronization Signal (PSS), also called NR-PSS, which allows for network detection with a high frequency error, up to tens of parts per million (ppm), and also provides a network timing reference.        Secondary Synchronization Signal (SSS), also called NR-SSS, which allows for more accurate frequency adjustments and channel estimation while at the same time providing fundamental network information in the form of a locally unique cell identity, also referred to as the Physical Cell Identity (PCI).        Tertiary Synchronization Signal (TSS), which provides timing information within a cell, e.g. between common control signaling beams transmitted in a cell and/or symbol number indications, e.g. allowing derivation of subframe borders. The existence of the TSS (and its format and specific usage if it is introduced) is still being studied in 3GPP.        
Together the PSS, SSS and possibly TSS are referred to as Synchronization Signal (SS).
The Synchronization Signal is transmitted together with a broadcast channel, referred to as Physical Broadcast Channel (PBCH) or NR-PBCH, carrying a small, but essential part of the system information, often referred to as the Master Information Block (MIB).
Together the SS and the PBCH form a structure denoted SS Block (SSB) which is periodically broadcast in a cell. FIG. 1 illustrates a possible format/structure of the SS Block.
The remaining parts of the system information is periodically broadcasted on one or more other channel(s). Some of the system information may not be periodically broadcasted at all, but may instead be requested (and transmitted) on demand.
NR Network (NW) Configuration
Depending on the deployment, beamforming may be used to distribute the SSB over the coverage area of a cell. Multiple SSBs are then aggregated to form an SSB burst where each SSB instance is beamformed in a certain direction, either to ensure coverage or to provide beam finding support for subsequent link establishment.
As mentioned above, for the purpose of improving coverage (or beam finding), the SS Block may be transmitted using beamforming in the form of a beam sweep including multiple beams which together cover the desired area. Another means for improving coverage is repetition of wide (even omnidirectional) beam transmissions. Both beam sweeping and repetition involves multiple transmissions. 3GPP has agreed on a structure for such multiple SS Block transmissions. A number of SS Block transmissions lumped together, i.e. transmitted in a tight series, are generally denoted “SS Burst” in the present specification. The notion of an “SS Burst Set” refers to a set of SS Bursts, typically with some non-zero interval between successive SS Burst transmissions (see FIG. 2, which illustrates the concepts of SS Burst and SS Burst Set). In 3GPP, the SS Blocks referred to herein are called SS/PBCH Blocks. The SS/PBCH Blocks have fixed locations defined in the 3GPP specification. A person skilled in the art would understand that when the expressions “SS Burst” and “SS Burst Set” are used within this specification, these expressions refer or relate to the SS/PBCH Blocks of 3GPP, when appropriate. An SS Burst may, for instance, consist of the beam transmissions of a full beam sweep. However, there may also be reasons for not including a full beam sweep in an SS Burst, for instance if the number of beams in the sweep is comparably high and a full beam sweep would take a longer time than allowed or desired for an SS Burst. In such a case, the beam sweep may be divided into multiple SS Bursts, e.g. forming an SS Burst Set. The periodicity of an SS Burst as well as the recurrence interval of a certain beam in a sweep may be configurable and deployment dependent. All the exact details around these time periods/intervals are not yet decided in 3GPP. Some values that are considered for these time periods include 5, 10, 20, 40, 80 and 160 ms. Different values as well as different configuration possibilities may be decided for different deployment scenarios.
Cell Re-Selection in NR
In addition to the UE energy saving state RRC_IDLE, where RRC stands for Radio Resource Control, which is used in LTE, 3GPP has introduced another energy saving UE state referred to as RRC_INACTIVE state. In RRC_IDLE state the context information related to the UE (the UE context) is deleted in the Radio Access Network (RAN) and the RAN has no knowledge of the UE and the UE's whereabouts. In the RRC_INACTIVE state, on the other hand, the UE context is kept in the RAN (e.g. gNB) and the RAN Core Network (RAN-CN) connection for the concerned UE is kept.
In these two states a UE monitors the relevant control signals from the network, including synchronization signal(s) and system information as well as potential page signaling. When performing such monitoring in a cell the UE is said to be camping on the cell.
A UE in RRC_IDLE or RRC_INACTIVE state constantly or repeatedly evaluates whether it is appropriate to remain camping on the current cell or whether camping on another cell, assumedly a neighbor cell of the current cell, would be better. Switching from camping on one cell to camping on another cell is referred to as “cell re-selection.” To assess the suitability of different cells to become (or remain) the cell the UE camps on (e.g. denoted the camping cell), the UE measures on the SS Block transmissions of the current and neighbor cells, in particular the SSS and possibly Demodulation Reference Signal (DMRS) on the PBCH. The signals a UE measures on to assess a cell's suitability and to determine whether cell re-selection should be performed are henceforth referred to as “cell re-selection measurement signals.” In addition to the measurements on the cell re-selection measurement signals, the UE may be guided by cell re-selection rules conveyed to the UE via the system information in the camping cell. Such rules may for instance contain thresholds (e.g. in terms of signal strengths or signal qualities) for cell re-selection and hysteresis to prevent a UE from constantly re-selecting between two neighboring cells.
As described above, the periodicity of the signals a camping UE uses for cell re-selection assessment may be quite long, resulting in long measurement periods when the UE monitors the downlink of neighbor cells, blindly searching for signals to measure on. Much of this time, potentially the major part of this time is likely to be wasted because there are no relevant signals transmitted in any of the cells neighboring the UE's current cell. This is costly from a UE energy consumption perspective and hence counteracts the energy-saving purpose of the RRC_IDLE and RRC_INACTIVE states.